[llvm] AMDGPU: Allow only VGPR wide sources in fp6/4/8 conversions (PR #127464)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

We should also have codegen tests that show SGPR to VGPR copies inserted in lowering 

https://github.com/llvm/llvm-project/pull/127464